Microwave antennas are primarily used for transmitting and receiving microwave radiation from free space. The shapes of microwave antennas depend upon their configuration: dish or horn shaped for single feed, and flat or conformed patch for multiple feed phased arrays. The finite size of these antennas creates appreciable side lobes which lead to performance degradation. These side lobes are the result of edge diffraction of the radiation from the feed. The diffraction spreads the radiation into unwanted directions and causes interference with other electronic systems. A proper edge treatment will reduce the strength of these side lobes and enhance antenna performance. Many methods have been suggested. The two most common are serrated edge and rolled back edge. The present invention is an improvement on both.
The edges of widely used microwave antennas have not been properly treated. These antennas have shapes which can be categorized as, horns, dishes, or patches. Two current methods of serrated edge and rolled back edge are closely related to the present invention. Both modify the characteristic of the antenna edges by adding skirts along the rim, yet still maintain the basic structure of the antennas. This form of modification is usually referred to as the edge treatment.
The theoretical foundations and designs for microwave antennas with serrated or rolled back edges are widely publicized and were intensively debated at the Annual Meetings and Symposiums of the Antenna Measurement and Techniques Association for at least past ten years. The supporters of both camp have repeatedly argued the advantage and superiority of these two distinctive designs.
There are considerable differences between these two designs. The serrated edge treatment simply extends the surface of a microwave antenna. The surface curvature remains the same, but the extended surface area is gradually reduced to zero during the extension. The controlling variable is the surface area in the edge diffraction reduction. The rolled edge treatment takes a different approach. While extending the edge, the surface curvature changes gradually and the added skirt as a whole is rolled back. The latter treatment emphasizes the control of the curvature variable.
The surface area and curvature of the added skirt are two independent variables which can be varied simultaneously or individually. The edge diffraction reduction is an optimization process. The serrated edge treatment emphasizes the importance of the added skirt area, and the rolled edge treatment emphasizes the skirt curvature. These two treatments are both single-variable optimization procedures.
A microwave antenna projects a traveling microwave onto an aperture in free space. The electromagnetic field at each point as define by the projection becomes a new source of a secondary spherical wave and is known as Huygens' wavelet. The envelope of all Huygens' wavelets emanating from the antenna aperture at any instant of time is then used to describe the transmitting electromagnetic radiation from the antenna at a later instant of time. The above mechanism is known as the famed Huygens-Fresnel Principle. Mathematically, this principle can be represented by the Rayleigh-Sommerfeld diffraction formula which is a Fourier type integration.
The aperture of any antenna must be finite in size. This restriction imposes a rectangular window on the Rayleigh-Sommerfeld diffraction formula for an untreated microwave antenna. It is well known in Fourier analysis that a rectangular window leads to high side lobes. These side lobes can be properly reduced by employing smooth tapered windows before evaluating the Fourier transformation. The edge treatment of microwave antennas corresponds to imposing a smooth tapered window onto the Rayleigh-Sommerfeld diffraction formula. The serrated and rolled edge treatments differ in methods of tapering. The former is restricted to the magnitude tapering of the electromagnetic field at the aperture of a microwave antenna, and the latter is mainly confined to phase tapering with little controls on the magnitude. The electromagnetic field has two independent components--magnitude and phase. Any abrupt change in either component will lead to high sidelobes. Both serrated and rolled edge treatments are restricted to a single component, neglecting the other. The abrupt change can not be optimally removed with either of these two methods. The present invention treats both two components simultaneously, hence provide a better optimum method than either of them, therefore leading to much better side lobe reduction and a smaller size of the added skirt.